1. Field of the Invention
This invention relates generally to quantitatively measuring the power distribution of an ultrasonic acoustic wave in a medium, and in particular to using schlieren imaging for quantitatively measuring the acoustic pressure, intensity, or power distribution of an ultrasonic wave in a medium.
2. Description of Related Art
Ultrasonic acoustic waves have many medical applications. For example, ultrasonic waves are commonly used to scan tissue and can be focused to destroy tumors. Accordingly, many types of medical equipment contain transducers which convert electrical energy into ultrasonic acoustic energy. For safety and efficiency, this medical equipment is regularly tested to determine the distribution of power in the ultrasonic waves. The tests must be sufficiently accurate and quantitative to indicate whether the equipment meets design specifications and legal requirements.
Prior art testing of medical ultrasound equipment commonly employs a probe which detects acoustic pressure caused by an ultrasonic acoustic wave propagating through a fluid. The probe is used to measure the acoustic pressure at a series of points in the fluid. The series of measurements indicates the acoustic pressure distribution from which power in the ultrasonic wave can be determined. This prior art testing has several drawbacks. One drawback is that a complete measurement of the power distribution of an ultrasonic wave requires many measurements and consequently a considerable amount of time. Another drawback is that the probe can interfere with the ultrasonic wave being measured and cause the measurements to be inaccurate.
Schlieren imaging has been used to qualitatively observe the propagation of ultrasonic acoustic waves through a transparent medium by observing the light diffracted from variations caused in the medium by acoustic pressure. A conventional schlieren system 100 is shown in FIG. 1 and includes a source 110 of parallel light 121 of wavelength .lambda., an immersion tank 120 which contains a transparent medium 122, and an optical system 130. When an ultrasonic acoustic wave having wavelength A propagates through medium 122, the ultrasonic acoustic wave causes a pattern of localized changes in pressure (which in turn causes a pattern of localized changes in the refractive index of medium 122). Light 121 from light source 110 is diffracted by the pattern of localized changes in the refractive index as light 121 passes through transparent medium 122.
Diffracted light 123 exits medium 122 at discrete angles .THETA..sub.m relative to the direction of incident light 121. The discrete angles .THETA..sub.m are approximately m(.lambda./.LAMBDA.), wherein m is an integer indicating the order of the diffracted light. Optical system 130 focuses diffracted light 123 using a spherical focusing mirror 131. A knife edge 132, that is positioned in a light beam 135 from spherical focusing mirror 131, blocks the zero-th order light, but portions of the first and higher orders of diffracted light are focused into an image of the regions in medium 122 which diffracted the light. The image therefore qualitatively shows the pattern of the ultrasonic acoustic wave.
Although prior art schlieren systems such as that shown in FIG. 1 provide qualitative images of ultrasonic acoustic waves, prior art schlieren systems have not provided measurements which are suitable for testing of medical equipment. The images formed are blurred, do not contain a precisely controlled fraction of the diffracted light, and contain too much background light (noise) to quantitatively measure diffracted light intensities from precisely defined regions of medium 122. Further, such prior art images do not give a usable quantitative measure of the variations in the refractive index, pressure, or power in the ultrasonic acoustic wave because the refractive index, pressure, and power are not linearly related to the measured diffracted light intensity. Accordingly, the prior art schlieren systems can not be used for testing the efficiency and safety of medical equipment such as ultrasound scanners.